Several growth factors that induce cellular mitogenesis and proliferation act through membrane-embedded G protein-coupled receptors (GPCRs). GPCRs couple to, and stimulate, heterotrimeric G proteins which, upon activation, dissociate to G.alpha. and G.beta..gamma. subunits. Both these molecules can transduce intracellular signals via activation of specific effector proteins. The intracellular signaling events leading to cellular proliferation following GPCR-activation appear to be transduced largely through the activation of p21.sup.ras (Ras) and subsequent activation of the p42 and p44 mitogen-activated protein (MAP) kinases. Growth factors which act through GPCRs, such as lysophosphatidic acid (LPA) via the LPA receptor and norepinephrine via .alpha.2-adrenergic receptors, have been shown to activate Ras and MAP kinase primarily through G.beta..gamma. (Koch et al, Proc. Natl. Acad. Sci. USA 91:12706 (1994)).
The last 194 amino acids (Gly.sup.495 -Leu.sup.689) of the bovine .beta.-adrenergic receptor kinase-1 (.beta.ARK-1) represent a specific and selective G.beta..gamma.-inhibitor (see FIG. 1 for amino acid sequence of .beta.ARK-1-(495-689) and a nucleic acid sequence encoding same, SEQ ID NOS:1 and 2). .beta.ARK-1 is a G.beta..gamma.-dependent, cytosolic enzyme which must translocate to the membrane where it can phosphorylate its receptor substrate by physically binding to the membrane-anchored G.beta..gamma. (Pitcher et al, Science 257:1264 (1992)). The peptide encoded by the plasmid designated .beta.ARK-1-(495-689) Minigene (which peptide is designated .beta.ARK.sub.CT) contains the specific G.beta..gamma.-binding domain of .beta.ARK-1 (Koch et al, J. Biol. Chem. 268:8256 (1993)). When cells are transfected with the .beta.ARK-1-(495-689) Minigene (that is, the .beta.ARK.sub.CT Minigene), or peptides containing the G.beta..gamma.-binding domain of .beta.ARK-1 are introduced into cells, several G.beta..gamma.-dependent processes are markedly attenuated including .beta.ARK-1-mediated olfactory receptor desensitization (Boekhoff et al, J. Biol. Chem. 269:37 (1994)), phospholipase C-.beta. activation (Koch et al, J. Biol. Chem. 269:6193 (1994)) and G.beta..gamma.-dependent activation of Type II adenylyl cyclase (Koch et al, Biol. Chem. 269:37 (1994)). These studies demonstrate that the .beta.ARK-1-(495-689) peptide (that is, .beta.ARK.sub.CT) is G.beta..gamma.-specific, that is, that it does not alter G.alpha.-mediated responses (Koch et al, Proc. Natl. Acad. Sci. USA 91:12706 (1994); Koch et al, Biol. Chem. 269:37 (1994)). A further study utilizing the .beta.ARK.sub.CT Minigene has demonstrated that the growth factor IGF-1, by binding to its specific receptor, activates the Ras-MAP kinase pathway via G.beta..gamma.. These results indicate that certain receptor-tyrosine kinase-mediated cascades include a G.beta..gamma. component, as do those for LPA and other agonists that activate classical GPCRs (Luttrell et al, J. Biol. Chem. 270:16495 (1995)).
The present invention is based, at least in part, on the observation that the .beta.ARK.sub.CT peptide mediates inhibition of G.beta..gamma. function in vivo and that, in smooth muscle cells, that inhibition is associated with a modulation of cell proliferation.